When manufacturing integrated circuit devices, the electrical characteristics of a given type of device may vary even though each device is produced using the same manufacturing steps. Accordingly, the variations of these electrical characteristics from device to device should be maintained within a relatively small range to maintain the interchangeability thereof. In particular, integrated circuit devices which produce frequency modulation signals should have a relatively uniform carrier and deviation of the frequency modulation signal. Accordingly, the complexity of manufacturing methods used to produce such devices may be increased in order to maintain a uniformity of electrical characteristics.
When producing integrated circuit frequency modulation devices according to the national televideo system committee (NTSC) standard which defines video signals for video cassette recorders (VCR), the frequency modulation signal should have a carrier frequency of 3.4 Mhz.+-.0.1 Mhz corresponding to a synchronization tip level of the input video signal. The deviation of the frequency modulation signal should have a frequency of 1.0 Mhz.+-.0.1 Mhz corresponding to a white peak of a video input signal. More particularly, these standards are provided for the video home system (VHS).
FIG. 1 illustrates a circuit according to the prior art which generates a frequency modulation signal. This circuit includes a carrier control part 2, a deviation control part 3, and a voltage controlled oscillator 4. The carrier control part 2 receives a video input signal Vin having a synchronization signal as well as video input information. The carrier control part 2 generates a first voltage in response to the variable resistor VR1 which corresponds to a synchronization tip level for the video input signal. The deviation control part 3 generates a second voltage in response to the variable resistor VR2 which corresponds to a white peak of the video input of the video input signal. The voltage controlled oscillator 4 generates the frequency modulation signal FM wherein the carrier of the frequency modulation signal has a frequency of 3.4 Mhz.+-.0.1 Mhz and a deviation of the frequency modulation signal has a frequency of 1.0 Mhz .+-.0.1 Mhz in response to the first and second voltages.
In order to ensure that the frequency modulation signal falls within the above-mentioned parameters, variable resistors VR1 and VR2 are provided at the periphery of the carrier control part 2 and the deviation control part 3. Accordingly, the frequency of the frequency modulation signal FM can be adjusted using the above-mentioned variable resistors. The adjustment of the variable resistors, however, may increase the time required to manufacture the video cassette recorder thus reducing the manufacturing costs thereof. This increased manufacturing time is due to the time required to manually adjust the variable resistors. Accordingly, there continues to exist a need in the art for improved frequency modulation circuits.